1. Field of the Invention
The present invention relates to a bearing device for a motor used for various types of OA apparatus.
2. Description of the Related Art
In recent years, a reduction in the cost of electronic apparatus employed in the field of, for example, office automation has advanced, and thus there has been a demand for an inexpensive motor for such electronic apparatus, which is the heart of a driving system thereof. The cost of the motor is greatly affected by the cost of a bearing element. A highly reliable miniature ball bearing is very expensive, while a relatively inexpensive oilless slide bearing is slightly less reliable than the miniature ball bearing. Thus, in order to achieve a reduction in the cost of the motor, there has been a demand for a highly reliable oilless slide bearing.
The structure of a bearing device for a conventional motor will be described below.
FIG. 6 shows an axial fan motor which is a typical example of a motor employing the above-mentioned oilless slide bearing. In FIG. 6, reference numeral 1 denotes a shaft; reference numerals 2 and 3 respectively denote a rotor frame side oilless slide bearing and an anti rotor frame side oilless slide bearing which rotatably support the shaft 1; reference numeral 4 denotes a thrust plate which axially supports loads in the thrust direction; reference numeral 5 denotes a snap ring for stably bringing the thrust plate 4 into contact with an end surface of the anti rotor frame side oilless slide bearing 3 to rotate the thrust plate 4 together with the shaft 1.
Reference numeral 6 denotes a stator boss for fixedly accommodating the oilless slide bearings 2 and 3; reference numeral 7 denotes a stator boss supporting portion for fixedly supporting the stator boss 6; reference numeral 8 denotes a rotor frame which is coupled to the shaft 1 and is thus rotatable together with the shaft 1; reference numeral 9 denotes a stator fixed to an outer peripheral portion of the stator boss 6. A reference numeral 10 denotes a field magnet adhered to an inner periphery of the rotor frame 8. The motor is rotated as a result of the magnetic repulsion which acts between the field magnet 10 and a driving coil 11 wound around the stator 9.
Reference numeral 12 denotes a fitting oil supplied in order to stabilize the initial lubrication between the end surface of the anti rotor frame side oilless slide bearing 3 and the thrust plate 4; reference numerals 13 and 14 respectively denote an oil thrower for preventing flow of the lubricant from the rotor frame side and a lubricant leakage preventing rubber cap for preventing leakage of the lubricant from the anti rotor frame side; and reference numeral 15 denotes a driving circuit for controlling commutation of the driving coil 11.
The operation of the thus-arranged conventional bearing device will now be described. When the rotor frame 8 and the shaft 1 rotate as a result of the magnetic repulsion which acts between the stator 9 and the field magnet 10, the inner-diameter surfaces of the rotor frame side and anti rotor frame side oilless slide bearings 2 and 3 slide against the outer-diameter surface of the shaft 1, and the end surface of the anti rotor frame side oilless slide bearing 3 slides against the thrust plate 4, thus causing a lubricant to well up due to expansion of and reduction in the viscosity of the lubricant in the oilless slide bearing, caused by the generation of a frictional heat as a result of sliding, and due to the pumping action of the oilless slide bearing itself. In the radial direction, the lubricant which has welled up is pushed into a narrow portion of a gap between the shaft 1 and the inner-diameter surfaces of the rotor frame side and anti rotor frame side oilless slide bearings 2 and 3 in a wedge-like form, generating a pressure in the lubricant. The generated pressure of the lubricant acts on the shaft 1 as a floating force, reducing the frictional resistance between the slide bearings 2 and 3 and the shaft 1. The floating force acts in such a manner that a lubricated condition in which no metal contact occurs is maintained over a long period of time.
In the axial direction, both the oil which has welled up and the fitting oil 12 swirlingly flow in a gap between the end surface of the anti rotor frame side oilless slide bearing 3 and the thrust plate 4, precluding a direct contact therebetween. Thus, a frictional resistance is reduced, and an excellent lubricated condition lasts over a long period of time.
However, in the above-described conventional structure, a force greater than the surface tension of the lubricant present near the thrust plate 4 and the snap ring 5 is exerted by the swirl flow of the lubricant which is generated by the slide of the end surface of the anti rotor frame side oilless slide bearing against the thrust plate 4 in order to axially support thrust load, and the lubricant thereby flows along the inner-diameter surface of the stator boss 6 and reaches as far as the contact between the stator boss 6 and the lubricant leakage preventing rubber cap 14.
If that happens, the lubricant may be absorbed by the rubber cap due to the characteristics of the material thereof. Alternatively, the lubricant whose viscosity has been reduced by the frictional heat may be present in the contact between the lubricant leakage preventing rubber cap 14 and the inner-diameter surface of the stator boss 6. That lubricant present in the contact may flow out to the outside of the motor along the boundary therebetween depending on the magnitude of the contact pressure between the rubber cap 14 and the stator boss 6.
The lubricant which leaks along the boundary may not only flow out to the outside of the motor along the gap between the inner peripheral surface of the stator boss supporting portion and the outer peripheral surface of the lubricant leakage preventing rubber cap, but also penetrate a gap between the stator boss and the stator boss supporting portion, and flows into the motor through the stator.
Such absorption and leakage of the lubricant occur whenever the motor is rotated. Leakage of the lubricant is stopped when the operation of the motor stops by the action of the surface tension of the lubricant and the capillary phenomenon of the oilless slide bearing. Due to the leakage of the lubricant which occurs during the rotation of the motor, the lubricant in the anti rotor frame side oilless slide bearing 3 is consumed greatly. Consumption of the lubricant in the anti rotor frame side oilless slide bearing 3 and the resulting damage to the anti rotor frame side oilless slide bearing 3 induce the damage to the rotor frame side oilless slide bearing 2, even though the oilless slide bearing 2 still contains a sufficient amount of lubricant in it, thus reducing the life of the entire bearing element.